Recombination of chromosome
The process, which produces recombinations of gene by interchanging of
corresponding segments between nonsister chromatids of homologous chromosomes,
is called recombination of chromosomes. It takes place in pachytene stage of
prophase I of meiosis. Crossing between the linked genes results in genetic
recombination.
According
to Bateson and Punnet, in Lathyrus odoratus 12 per cent of the test cross
progeny were recombinants. Recombination between two genes is expressed in
percentage. It is called recombination frequency. Gene pairs that had very low
percentage of recombination are known as tightly linked genes. The gene pairs
with higher percentage are termed as loosely linked genes. For example, 12 per
cent of the test cross progeny were recombinants. They showed a different
linkage of alleles than their parents. The percentage recombination is
determined by dividing the number of recombinant offspring by the total number
of offspring. In the figure 3.4, the linkages of the parents were B with L and
b with l. The recombinant offspring are B with l or b with L.
Chromosomes
Chromosomes are the physical carriers of genes, which are made up of DNA
and associated proteins. The term chromosome was introduced by Waldeyer in
1888. Chromosomes occur in all the living organisms. The bacterial chromosomes
are circular. It has closed circular DNA. Linear chromosomes are found in
eukaryotes. Bridges in 1916 was the first to prove that the genes are carried
on the chromosome.
Linkage
and crossing over
The tendency
of genes or characters to be inherited together because of their location on
the same chromosome is called linkage. Many hybridization experiments were
conducted both on plants and animals based on Mendel's work. The results of
certain dihybrid crosses did not confirm the law of independent assortment. It
states that the inheritance of genes of each pair in a dihybrid during gamete
formation is independent of the other.
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